A 3-year-old boy watched in horror as his mother was murdered in a Bronx home invasion – in which three gunmen, hunting for a drug stash in the wrong apartment, fatally shot the mom, her boyfriend and another man, police said yesterday.

The bloody attack began at 11 p.m. Tuesday in an apartment at 2427 Webster Avenue in Fordham, which neighbors said Delsia Tejada shared with her boyfriend, Miguel Aquino, who was not the father of her child.

The three gunmen were looking for 10 kilos of cocaine when they pushed their way into Apartment E8 with guns and bound the couple and another man and woman who were visiting, cops said.

The thugs made a futile attempt to beat the location of the drugs out of Aquino – not realizing they were in the wrong apartment.

They told Aquino they would begin shooting the others if he didn’t produce the drugs. Then they carried out the threat, killing Tejada, Aquino and Elvis Heartley, one of the visitors, before fleeing.

Joanne Heartly, who was also visiting the apartment, was hit in the shoulder and managed to play dead and escape after the killers left.

She ran to a next door neighbor for help.

“She had blood on her face, and she was holding her shoulder like she had been injured,” said Robert Marsh, 64, who took the woman in and called 911.

Marsh later witnessed cops take Sehing Tejada, the terrified tot, from the apartment – after they found him beside his mother’s lifeless body.

“He wasn’t crying, he seemed like he was in shock,” Marsh said.

One of the alleged gunmen was later arrested at a nearby hospital, where he sought treatment for a gunshot wound apparently inflicted by one of his partners during the shooting.

It wasn’t clear whether the shot was accidental or deliberate.

The man, Ricky Curet – who was arrested for murder on his 34th birthday yesterday – has 18 prior arrests and 14 convictions, sources said.

He served five years for a 1992 weapons-possession conviction for which he was released in April 1997, records show.

A month after his release, he was sentenced to a two-to-four-year sentence on a drug charge, for which he was released in September 2004.

Curet allegedly admitted to a role in the attack but claimed his partner, Noel Bruno, 26, was the triggerman, authorities said.

Bruno was later arrested, while a third suspect remained at large.

Residents and workers at the building said they saw no evidence of drug dealing at the apartment. A search by police drug dogs confirmed there were no drugs.